Method of manufacturing a microbalance

ABSTRACT

22A temperature and/or pressure compensated microbalance is disclosed. Temperature compensation is achieved by applying heat to at least a part of the microbalance, measuring a temperature-dependent variable, and controlling the amount of heat applied to the microbalance to keep the temperature-dependent variable substantially constant. In one embodiment, the heat is applied to the microbalance by passing electrical current through a resistive element provided on or embedded in the oscillating element of the microbalance. Pressure compensation is achieved by taking into account the variation in the mass or density of fluid passing through the microbalance. Various materials and methods of construction are also disclosed, including micro-machining and electroforming.

BACKGROUND OF INVENTION

[0001] Instruments used to measure a particular parameter may beaffected by the variation of other parameters. For example, measurementof the mass of material deposited on a microbalance may be adverselyaffected by a variation in temperature or pressure.

[0002] A microbalance, examples of which are described in U.S. Pat. Nos.3,926,271 and 4,391,338, typically comprises an oscillating elementmounted with one end fixed and the other end free. The free endtypically has a filter (or other mass-receiving element) mountedthereto. The oscillating element may also be hollow, and in such a casea fluid is typically drawn through the filter and through theoscillating element, thereby to trap suspended particles within thefluid in or on the filter. The resulting increase in the mass of thefilter decreases a resonant frequency of the oscillating element. Thedecrease in the resonant frequency of the oscillating element is relatedto the increase in mass of the filter, which in turn is equal to themass of the suspended particles trapped in the filter. Because theoscillating element has the ability to continually indicate the mass ofthe suspended particles it is ideal for indicating the change in mass ofthe trapped or suspended particles in near real time or over a measuredperiod of time.

SUMMARY OF INVENTION

[0003] As the temperature of the microbalance”s oscillating elementchanges, the resonant frequency of the oscillating element changes, eventhough the mass on the filter substrate secured to the oscillatingelement may remain unchanged. As the measured mass is based on theresonant frequency, an error is introduced in the mass determination.This temperature sensitivity results mainly from a change in the modulusof elasticity of the material, from which the oscillating element ismade, as the temperature changes.

[0004] One way of addressing the concern of the temperature sensitivityof the microbalance, or other instrument, is to select a material ofconstruction that has minimal sensitivity to changes in temperature. Fora microbalance, great care can be applied to the formulation of theglass from which the oscillating element is made, thereby to attain thedesired characteristics while attempting to optimize manufacturabilityand minimize the temperature sensitivity of the desired variables. Inparticular, one way of reducing the temperature sensitivity of themicrobalance is to use a shaped oscillating element made of a glasshaving a low temperature coefficient of elastic modulus. In the end,compromises must be made at the expense of the accuracy,manufacturability and cost of the entire system.

[0005] According to one aspect of the invention, the temperaturesensitivity of an instrument, for example the microbalance describedabove, is reduced by maintaining the instrument, or at least atemperature-sensitive element thereof, at a constant temperature. Thisis achieved by: applying heat to the instrument; measuring a parameterthat is indicative of the temperature of the instrument; and controllingthe amount of heat applied to the instrument to maintain the measuredparameter substantially constant.

[0006] For example, this control may be accomplished by affixing aresistive heater on the oscillating element of the microbalance. Theresistive heater can be wound onto the oscillating element, vacuumdeposited, or applied by any other means. Some glass formulations allowembedding platinum heater windings directly within the glass. Radiant orother types of heating, such as convective and conductive, can also beemployed. A radiant heater would be positioned appropriately next to oraround the oscillating element to provide heat thereto. The parameterthat is used to control the amount of heat supplied to the heater can bethe resistance of the heating element (which is dependent on thetemperature), or the output of an appropriately positioned temperaturesensor.

[0007] A further error in the use of microbalances having hollowoscillating elements is caused by temperature and/or pressure changes inthe fluid located within the cavity of the hollow oscillating element.As the temperature or pressure of the fluid within the cavity of thehollow oscillating element varies, so will the density of the fluid.Assuming that the interior volume of the cavity of the hollowoscillating element remains substantially constant, a variation in thedensity of the fluid will result in a variation of the mass of the fluidlocated in the cavity of the hollow oscillating element. That is, theeffective mass of the oscillating element will vary with temperatureand/or pressure changes in the fluid located therein. This variation inthe effective mass of the oscillating element will in turn affect theresonant frequency of the oscillating element.

[0008] The pressure-dependent error frequently manifests itself as aperceived negative mass over time. As the filter element loads up withparticulate or other forms of flow-impeding elements, the pressurewithin the cavity of the hollow oscillating element will decrease due tothe increased resistance of the filter element. As the pressure of thefluid decreases, so does the density, reducing the mass of the column offluid within the cavity of the hollow oscillating element. This in turnwill increase the resonant frequency of the oscillating element,indicating a false reduction in the mass of the filter element and itsentrapped matter.

[0009] Similarly, if the fluid gets colder, the density of the fluidcolumn in the cavity of the hollow oscillating element will increase,increasing the mass within the cavity of the hollow oscillating element.The resonant frequency of the oscillating element will becorrespondingly lower, thereby indicating an erroneously high mass. Thereverse is true if the fluid temperature increases.

[0010] The compensation for the variation in temperature or pressure canbe performed as follows, using an idealized gas as a fluid, with Boyle”slaw (PV=nRT) to approximate the behavior of the gas. It will beappreciated that other models and equations can be used for performingthe pressure/temperature/density compensation, and that other modelsand/or equations can be used to represent the dynamics of the instrumentin question.

[0011] Referring to FIG. 1, the principle of operation of a microbalance10 can be represented by m=k/f², where m is the mass in grams, f is thefrequency in Hz, and k is the spring constant in g*Hz².

[0012] For a particular microbalance, the spring constant can bedetermined by using two values of m one for “zero”mass (i.e. the systemmass only), and one for an additional mass added to the “zero” mass. Thesystem mass is of course not actually zero—we “tare” the system mass outfor purposes of convenience, much like a post office scale is zeroedbefore the letters are placed in a box on the scale. This ensures thatthe mass of the letters only is considered, and not the mass of the box.

[0013] The equation for the spring constant can be derived asfollows:k=(m₁m₀)/(1/f₁ ²1/f₀ ²) Using exemplary values of m₁=0.075 g,m₂=0, f₁=250 Hz and f₀=311.314 Hz yields a value of k=13,200 g*Hz²,obtained at a temperature of 20° C. and 29.92 inHg, with the fluid beingan air mixture. At this temperature and pressure, the air mixture has adensity of 1.200 g/l.

[0014] The actual system mass at this temperature can now be determinedby substituting the determined value of k and the observed value of ffor the “zero” mass condition. Using these two values, we arrive at asystem mass of 0.136199 g. Part of this system mass results from thecolumn of air in the cavity of the hollow oscillating element.

[0015] As mentioned, the mass at the “zero” condition (the system massonly) includes a mass component derived from the mass of the fluidcolumn in the cavity of the hollow oscillating element at the particularfluid density. The effect of the fluid column on the system mass can bedetermined by measuring the “zero” or system mass at a different fluiddensity, as follows.

[0016] Repeating the test described above at a reduced air pressure of20.00 inHg the new density (from Boyle”s law) isrho=rho₁P₁/P*T/T₁=0.8021 g/l The observed frequency at 20 inHg is 311.4Hz, which indicates a mass of m=k/f²=0.136124 g This new system mass,and its associated change in frequency, have resulted from the decreasein the fluid density (and hence the mass of fluid in the cavity of thehollow oscillating element). The change in mass, delta_m=0.1361990.136124=0.000075 g, which has occurred as a result of a change ofdensity, delta_rho, of 1.20 g/l 0.8021 g/l=0.3979 g/l.

[0017] The “active” volume, V, (i.e. the effective volume thatcontributes to the variation in the frequency) can be determined asfollows: V=delta_m/delta_rho=0.000075/0.3979 =1.884896E-4 l.

[0018] Using the parameters determined above, a compensation for avariation in mass of the fluid column of the cavity of the hollowoscillating element can be performed. One exemplary way of doing this isto edit the basic equation k=(m₁m₀)/(1/f₁ ²1/f₀ ²) to allow for thevariation in the mass of the fluid column within the cavity of thehollow oscillating element. Each of the masses in this equation can berepresented as a sum of the oscillating element mass and the fluidcolumn mass. That is, m_(n)=m_(nm) ^(m) _(nF), where m_(nm) is theoscillating element mass and mis the fluid column effective mass or“active” mass. The effective mass can be determined by multiplying thedensity by the “active” volume, which is determined as shown above.Thus, m_(1F)=V*rho_(1F). The variable rho_(1F) is a function of T_(1F)and P_(1F) as follows: rho_(1f)=rho_(S)*P_(S)/P_(1F)*T_(1F)/T_(S) Wherethe subscripts s refer to density, pressure and temperature at standardor reference conditions.

[0019] Substituting rho_(1f) if into the equation for m_(1F) we get:m_(1F)=V*rho_(S)*P_(S)/P_(1F)*T_(1F)/T_(S) Similarly,m_(0F)=V*rho_(S)*P_(S)/_(P0F)*T_(0F)/T_(S) Therefore,m₁−m₀=(m_(1m)+V*rho_(S)*P_(S)/P_(1F)*T_(1F)/T_(S))(m_(0m)+V*rho_(S)P_(S)/P_(0F)*T_(0F)/T_(S))m₁−m₀=(m_(1m)m_(0m))+V*rho_(S)*P_(S)/T_(S)*(T_(1F)/P_(1F)−T_(0F)/P_(0F))We now substitute these mass equations into a rearranged k=(m₁m₀)(1/f₁²1/f₀ ²), as follows: (m₁m₀)=k(1/f₁ ²1/f₀²)(m_(1m)m_(0m))+V*rho_(S)*P_(S)/T_(S)*(T_(1F)/P_(1F)−T_(0F)/P_(0F)=k(1/f₁ ²1/f₀ ²)m_(1m)=m_(0m)+k(1/f₁ ² original equation, we can see thatthe term V*rho_(S)*P_(S)/T_(S)*(T_(1F)/P_(1F)−T_(0F)/P_(0F)) providesthe compensation for the variation in temperature/pressure/density ofthe fluid column in the cavity of the hollow oscillating element. If thepressure and temperature remain unchanged, the term(T_(1F)/P_(1F)−T_(0F)/P_(0F))=0, and the equation reverts to theoriginal equation of m

[0020] But if the temperature and/or pressure vary, this term willprovide an adjustment.

[0021] In the practical application of the invention, T_(1F), P_(1F),T_(0F), P_(0F), f₁, f₀ are observed using appropriate sensors as will bedescribed in more detail below. The spring constant k is determinedalong with a known mass at conditions P_(0f) and T_(0f). The resultingobserved f₀ allows for the calculation of k.

BRIEF DESCRIPTION OF DRAWINGS

[0022] Exemplary embodiments of the invention will now be described withreference to the attached drawings, in which:

[0023]FIG. 1 is a side view of an idealized microbalance forillustrating the principles of operation thereof;

[0024]FIG. 2 is a sectional side view of a microbalance in accordancewith the present invention that is made utilizing an electroformingfabrication process;

[0025]FIG. 3 is sectional side view of a microbalance in accordance withthe present invention, that is made utilizing a glass fabricationprocess, showing an exemplary installation and related equipment;

[0026]FIG. 4 is a sectional view of the microbalance through the planeX-X shown in FIG. 2 and FIG. 3;

[0027]FIG. 5 is a sectional view of the wall of the microbalance of FIG.3 showing one configuration of the heating elements used to providetemperature control;

[0028]FIG. 6 is a sectional view of the wall of the microbalance of FIG.3 showing an alternative configuration of the heating elements used toprovide temperature control;

[0029]FIG. 7 is a sectional view of the wall of the microbalance of FIG.3 showing an alternative configuration of the heating elements used toprovide temperature control;

[0030]FIG. 8 is a sectional view of the wall of the microbalance of FIG.2 showing an alternative configuration of the heating elements used toprovide temperature control;

[0031]FIG. 9 is a schematic view of a data processor and relatedcomponents for use with the microbalances of FIGS. 1 to 8, 10, 11, and12;

[0032]FIG. 10 is a layout of a micro-machined equivalent of the innerworkings of the microbalance,

[0033]FIG. 11 is a sectional side view of the packaged micro-machinedequivalent of the microbalance; and

[0034]FIG. 12 shows an oblique view of the micro-machined microbalanceto clarify the geometry of the microbalance.

DETAILED DESCRIPTION

[0035] Referring now to FIGS. 2 and 3, a microbalance 10 comprises anelastic oscillating element 12, a base 14 and a resistive heater 16.

[0036] The oscillating element 12 may be made of known materials used inthe manufacture of microbalances, but it may also be made of othermaterials such as nickel alloys, inconel, quartz, and quartz-glassalloys. For example, a nickel-cobalt alloy may be used for increasedstrength and decreased temperature coefficient of elasticity. Byapplying the temperature compensation described herein, it is possibleto use materials with less-restricted temperature dependencies of themodulus of elasticity, since the invention reduces the sensitivity ofthe instrument to external temperature variations.

[0037] Additionally, the oscillating element 12 may be manufacturedusing the electroforming manufacturing method. The electroforming methodis similar to the method utilized for chrome plating automobile bumpersexcept that an appropriately shaped form is used on which to plate thematerial. The use of electroforming materials from which to make theoscillating element 12 will inherently result in a lower temperaturedependence over the normal operating temperature range of the systemthan many of the glass compositions available, since the Young's modulusof elasticity of the electroforming materials is typically a few ordersof magnitude larger than the glass compositions available. Additionally,oscillating elements made from an electroformed material will be morerugged than glass elements, potentially lending the subject device towider use.

[0038] The resistive heater 16 is wound around, or affixed to, theoscillating element 12, and is coupled to heater control electronics 19as shown in FIG. 3. The resistive heater 16 may be covered with aninsulating material 17, or may be embedded in the oscillating element12. Further alternative configurations of the resistive heater 16 aredescribed below with reference to FIGS. 5 to 8. In use, the heatercontrol electronics applies a current to the resistive heater, therebyto apply heat to the oscillating element 12. The heater controlelectronics 19 also measures the resistance of the resistive heater, andcontrols the current applied to the resistive heater 16 to maintain theresistance thereof substantially constant. The resistance of theresistive heater 16 is a measure of the temperature of the resistiveheater, and so by maintaining the resistance constant, the temperatureof the resistive heater is held substantially constant. Of course, othertemperature-dependent variables could be used to control the temperatureof the oscillating element 12 of the microbalance 10. For example, theoutput of an appropriately positioned temperature sensor could be used.

[0039] The oscillating element 12 is mounted at one end thereof to thebase 14, and the other end thereof is free to vibrate. As can be seen inFIG. 4, the oscillating element 12 has an elliptical cross-section inthe illustrated embodiment, but other cross-sections may be used. Theelliptical cross section results in the primary/lowest resonantfrequency of the oscillating element 12 being in a predictable path,that is, along the minor axis. The means for exciting the oscillatingelement 12, and for measuring the resulting vibration, are accordinglyalso located along the minor axis of the ellipse.

[0040] Mounted towards the upper end of the oscillating element 12 is adisc 20 of magnetic or iron alloy material that is used to couple theexcitation force to the oscillating element 12 when acted upon by anelectromagnet 22 (FIG. 3) under control of EM vibrator electronics 24.Mounted to the oscillating element 12 opposite to the disc 20 is amagnetic or iron alloy disc 26 (typically the same as disc 20) thatunder vibration of the oscillating element 12 causes a fluctuation inthe magnetic field in an electromagnet, or position sensing transducer,28, which is detected by frequency measuring electronics 30. As the namesuggests, the frequency measuring electronics 30 measure the resonantfrequency of the oscillating element 12 of the microbalance 10. It willof course be appreciated that other structures and methods may be usedfor exciting the oscillating element 12 and for measuring the resonantfrequency of the oscillating element 12 of the microbalance 10. Forexample, optical devices and methods may be used to measure thefrequency of the oscillating element 12 of the microbalance 10.

[0041] The oscillating element 12 is hollow, with a cavity 18 definedtherethrough. In use, a fluid is drawn through the oscillating elementfrom the free end to the fixed end, and out through a passage defined inthe base 14. As can be seen from FIG. 3, this fluid flow is generated bya vacuum supply or pump 32 that is in fluid communication with thecavity 18.

[0042] The electroformed oscillating element 12 of FIG. 2 is mounted tothe base 14 by means of a number of screw fasteners 34 that clamp alower flange of the oscillating element 12 to the base 14 via a ring 33.An O-ring 36 ensures a fluid-tight seal between the oscillating element12 and the base 14.

[0043] The glass oscillating element 12 of FIG. 3 is mounted to the base14 by means of an adapter 35. Surrounding the oscillating element ofFIG. 3 is a housing 39 that is held together and to the base 14 by anumber of screw fasteners 41. A similar housing (not shown) is providedin use for the microbalance 10 of FIG. 2.

[0044] The mounting configuration of FIG. 2 differs from the mountingconfiguration of FIG. 3 because the oscillating element 12 of FIG. 2 isa metallic electroformed part (see further below) while the oscillatingelement 12 of FIG. 3 is made of a quartz glass or a quartz alloy. Thetemperature coefficient of expansion of the quartz glass or quartz alloyis typically different from the temperature coefficient of expansion ofthe base 14, which can cause breakage of a quartz glass/alloyoscillating element 12 under temperature variation. To reduce theeffects of this thermal mismatch, the oscillating element 12 of FIG. 3is coupled to the base using the adapter 35, that is made of Kovar orother material that has a similar temperature coefficient of expansionas the quartz glass/alloy oscillating element 12.

[0045] Referring again to both FIGS. 2 and 3, the base 14 has a bore 38defined therethrough, and a connector 40 that provides a connection towhich the vacuum supply or pump 32 can be connected. A temperaturetransducer 42 is mounted in the base with its sensitive element incommunication with the bore 38. The temperature sensor 42 in conjunctionwith temperature transducer electronics 44 is used to determine thetemperature of the fluid in the bore 38. Similarly, a pressuretransducer 46 and associated pressure transducer electronics 48 are usedto determine the pressure of the fluid in the cavity 18 defined by theinterior of the hollow oscillating element 12.

[0046] In use, a filter holder 50 is mounted to the free end of theoscillating element 12. The filter holder 50 provides a fixture forpositioning a replaceable filter 52 so that the fluid that is drawn intothe oscillating element 12 passes through the filter 52. As the fluid isdrawn through the filter, particulate matter 54 in the fluid becomestrapped in or on the filter 52. It is the measurement mass of theparticulate matter 54 trapped by filter that is the purpose of themicrobalance 10. It can be appreciated that the filter 52 must be firmlyaffixed to filter holder 50 to prevent the filter 52 from movingrelatively to the filter holder 50. This can be accomplished by securingthe two elements together with any appropriate securing means (e.g.glue, epoxy or ultrasonic welding) compatible with the specificapplication.

[0047] FIGS. 5 to 8 show enlarged views of exemplary alternativeconfigurations of the resistive element 16.

[0048]FIG. 5 shows a mechanically or electrochemically depositedresistive element 16 with an insulating material 17. The resistiveelement 16 can be applied using chemical vapor deposition or anysuitable plating process appropriate for the selected materials ofconstruction. The insulating material 17 can be any suitable material,but may for example be an ML epoxy applied by spraying or with a paintbrush, a Pyralene PVD (physical vapor deposition) insulating material,or a titanium carbo-nitride layer deposited by chemical vapordeposition. FIG. 6 shows a mechanically wound resistive element 16 withinsulating material 17.

[0049]FIG. 7 shows a resistive element 16 that is mechanically woundwhile the glass from which the oscillating element 12 is made is heatedto the soft working temperature of the glass. The resistive element canalso be heated by running current through it to facilitate embedding.This embeds the resistive element 16 in the oscillating element 12 andthereby eliminates the need for the insulation shown in FIG. 5, FIG. 6,and FIG. 8. Embedding the resistive element in this way provides goodthermal contact between the resistive element 16 and oscillating element12.

[0050]FIG. 8 shows a cross section of a metallic electroformedoscillating element 12. As the element is fabricated of a electricallyconductive material, it is necessary to provide an insulation layer 56that prevents shorting of the resistive element 16 to the oscillatingelement 12. The insulation layer 56 can be applied by any chemicalplating method. The insulating layer 56 can be any suitable material,but may for example be an ML epoxy applied by spraying or with a paintbrush, a Pyralene PVD (physical vapor deposition) insulating material,or a titanium carbo-nitride layer deposited by chemical vapordeposition. The resistive element 16 may be mechanically wound or usingchemical vapor deposition or any suitable plating process, such as threedimensional photolithography, appropriate for the selected materials ofconstruction. The insulating material 17 can be any suitable material,but may for example be an ML epoxy applied by spraying or with a paintbrush, a Pyralene PVD (physical vapor deposition) insulating material,or a titanium carbo-nitride layer deposited by chemical vapordeposition. When electroformed, a suitable form is provided. The formcan, for example, be injection molded using appropriately designedinjection-molding tooling. The form can be made of a plastic such asPVC, a nylon or similar plastic, or even an injection molded or castmetal. As electroforming requires the form to be electricallyconductive, plastic forms are typically coated with aluminum or silverutilizing methods similar to coating model car parts such as bumpers,such as electrolysis or electroplating processes. The form can howeverbe made using any suitable manufacturing method (e.g. 4 axis machining,travelling wire EDM) or material.

[0051] The form typically defines the interior profile of theoscillating element 12. The oscillating element 12 is formed on theoutside of the form, using conventional electroforming methods. When theoscillating element 12 has been formed, it is removed from the form, forexample using a jack-screw assembly, where the jackscrew engages theplated and non-plated parts to jack them apart, or by using a chemicalsolution (e.g. a caustic soda solution) to dissolve the coating on theform or the entire form itself.

[0052]FIG. 9 shows an exemplary data-processing configuration that maybe used with the microbalance described above. As can be seen from thefigure, a data processor 60 can be provided to receive information fromand/or to control the various control electronics described above, aswell as vacuum pump control electronics 62.

[0053] The data processor 60 may be a general-purpose computer or adedicated microprocessor, or any other computing device havingsufficient computing capabilities to operate the microbalance. It willalso be appreciated that some or all of the control and computingfunctions shown separately in FIG. 9 may be integrated into one or morecontrol or computing devices. Similarly, the particular interconnectionsbetween the devices may be varied, or may not be present at all. Forexample, the heater control electronics 19, the EM vibration electronics24 and the vacuum pump control 62 may be freestanding units notconnected to other units or the data processor 60. Similarly, whentemperature control only is used, the pressure control electronics 48may not be present at all. It will thus be appreciated that manyconfigurations may be used in operation of the invention.

[0054] Referring now to the attached figures, in particular FIGS. 3 and9, in use the heater control electronics 19 is actuated to provideelectrical current to the resistive element 16 to heat the oscillatingelement 12. The heater control electronics monitors the resistance ofthe resistive element 16, and controls the amount of electrical currentprovided to the resistive element 16 to maintain the resistance of theresistive element 16 substantially constant. As mentioned above, theresistance of the resistive element 16 is dependent on its temperature,any by maintaining its resistance substantially constant, thetemperature of the oscillating element 12 of the microbalance 10 ismaintained substantially constant. Alternatively, the output of anappropriately located temperature sensor can be used to provide thefeedback used to control the temperature of the oscillating element 12of the microbalance 10.

[0055] The particular control temperature used varies according to thecircumstances, but will typically be some amount (e.g. 5 to 10 degreesCelsius, or more) above the maximum expected ambient temperature thatwill be encountered by the microbalance, to ensure that the control ofthe temperature can be maintained. For example, a temperature of 50degrees Celsius could be used for typical atmospheric conditions.

[0056] In an implementation where only the temperature of themicrobalance is to be controlled (as opposed to also compensating forthe variation in the mass of the fluid in the cavity of the hollowoscillating element 12 the oscillating element 12, of FIG. 3, is nowexcited using the EM vibration electronics 24 and electromagnet 22 toapply an excitation force to the disc 20. The frequency of the resultingvibration is measured by the frequency measuring electronics 30 usingthe signal generated in the electromagnet, or position sensingtransducer, 28 by the movement of the disc 26. The vacuum pump 32 isthen actuated, which reduces the fluid pressure in the oscillatingelement 12. This in turn draws fluid through the filter 52. As the fluidis drawn through the filter, particulate matter 54 is trapped thereby.The increased mass of the filter element results in a reduction of theresonant frequency of the oscillating element 12. As discussed in theSummary of the Invention, this change in resonant frequency is used todetermine the mass of the particulate matter. This determination istypically done in the data processor 60 of FIG. 9.

[0057] If the density of the fluid is also to be compensated for (note,density compensation can also be done without temperature control),initial and subsequent pressure and temperature measurements are takenusing the pressure transducer electronics 48 and pressure sensor 46 andthe temperature transducer electronics 44 and temperature sensor 42.These measurements are then provided to the data processor 60, whichcompensates for the variation in density (mass) of the fluid in thecavity of the hollow oscillating element 12 as described above in theSummary of the Invention. As such it can be appreciated that an improvedmeasurement of the change in mass may be achieved in near real time.

[0058] Hereinafter, the geometry and operational features and theory ofthe oscillating element described fully above shall be referred to as“subject oscillating element.”Micro-machined pressure transducers andtemperature transducers have been widely used since the late 1980's,even in toys and consumer products. Micro-machining is conventionallyaccomplished using tools similar to those utilized in semiconductorfabrication, and is accomplished by using masks, photoresist, ion-beametching, X-ray photolithography, or other semiconductor fabricationtechniques, to selectively etch away parts of a silicon wafer (or otherequivalent base material) or add new structural layers, thereby to formthe mechanical and electromechanical devices.

[0059] The use of micro-machining to manufacture polysilicon resonantmicrostructures is also known. Refer, for example, to a paper deliveredat the IEEE Micro Electro Mechanical Systems Workshop, Salt Late City,Utah, Feb. 20-22, 1989 entitled “Laterally Driven Polysilicon ResonantMicrostructures” by William C. Tang, Tu-Chong H. Nguyen, and Roger T.Howe, University of California at Berkeley. Dept of ElectricalEngineering and Computer Sciences, 1989. Hereinafter, said paper shallbe referred to as “Resonant Microstructure Paper,” the disclosure ofwhich is incorporated herein by reference as if explicitly set forth.This paper describes the geometry of a structure, with modifications,that can accomplish essentially the same functionality as the subjectoscillating element.

[0060] A micro-machined equivalent of the described subject oscillatingelement can be designed either as a “permanent, multi-use,” “throw away”or “single use” device that encompasses all of the elements shown inFIG. 3 except for the electronics described in FIG. 9. The heater,pressure transducer, temperature transducer and of course the EMvibrating means and motion sensing (provided by the electrostatic combdrive and sensing described in the Resonant Microstructure Paper) areincluded in the micro-machined device. The micro-machined version of thesubject oscillating element would oscillate laterally, just like thesubject oscillating element, except there would be no arc described inthe micro-machined version because the micro-machined version isconstrained to oscillate in a single plane compared to a typical 2″ to5″ long oscillating element 12 shown in FIG. 3. The particulate matterwould impact directly on the laterally resonating plate and indicate thechange in mass over time using essentially the same transfer functionsas those for the subject oscillating element. The operation of themicro-machined microbalance is substantially as described above for themicrobalances manufactured using other techniques. Additionally, themicro-machined microbalance may also be operated without temperature orpressure compensation as appropriate.

[0061]FIG. 10 shows an exemplary version of the micro-machinedequivalents for the subject oscillating element using the same numberingscheme as FIG. 3. Referring to FIGS. 10 to 12, the micro-machinedmicrobalance comprises a base 14 that supports a dual folded beamsuspension 12. The dual folded beam suspension 12 supports an impactionplate 50 (the equivalent of the filter holder and filter substrate).Alternatively, a filter substrate and a filter may be provided insteadof the impaction plate. The dual folded beam suspension 12 is connectedto comb structures 20 and 26. The dual folded beam suspension 12 isdriven by the complementary comb structure 22 operating on the combstructure 20, and the movement of the dual folded beam suspension 12 issensed by the complementary comb structures 26 and 28. The ResonantMicrostructure Paper describes various additional drive and sensingmeans that may be implemented by those of ordinary skill in the art.

[0062]FIG. 11 shows an exemplary cross section of the micro-machinedelement in its package 63 with the dual folded beam suspension 12connecting the impaction plate 50 and 52. As in the subject oscillatingelement we have connector 40 connected to the vacuum source or pump 32.The base(s) 14 are connected to the package 63 to define the completeassembly.

[0063]FIG. 12 shows an exemplary oblique view of the micro-machinedelement to clarify the similarities of this design to the subjectoscillating element. We can see that the base 14 connects to thestructure leaving the impaction plate 50 52 free to oscillate on thedual folded beam suspension 12, the equivalent of the subjectoscillating element item 12, driven by the electrostatic comb drive at22 and measured by measuring the capacitance using a dc bias andmeasuring the short-circuit current through the time-varying combstructure at 28 through the movement of the comb structure at 26.

[0064] As one example of the fabrication process for micro-machining thenecessary geometry for a micro-machined version of subject oscillatingelement can be accomplished using the four-mask process described in theResonant Microstructure Paper, adapted as necessary. An advantage of thedescribed four-mask process is that all the critical features aredefined with one mask, eliminating the errors due to mask-to-maskmisalignment. This four-mask process is used in the vast majority ofmicro-machined elements.

[0065] The fabrication process begins with the deposition standard POCL₃blanket n+ diffusion. Next we deposit an LPCVD (Low Pressure ChemicalVapor Deposition) silicon nitride (Si₃N₄) layer. The first mask definesthe contact windows, which are etched using a combination of plasma andwet etching. The first Polysilicon layer is then deposited using LPCVD.This layer is masked with the second mask and etched to provideinterconnection to the n+ diffusion, standoff bumps are defined toprevent suspended micro elements from sticking. An LPCVD layer is thendeposited as a sacrificial phosphosilicate glass (PSG) layer that willbe removed to create the necessary cavities that define the essence ofwhy micro-machining has gained wide use. The third mask defines theanchors for the microstructures. The next deposition is the LPCVDPolysilicon structural layer. We then dope this layer with PSG to dopethe polysilicon symmetrically from the top and the bottom layers of PSG.We then stress anneal the entire structure at a temperature lower than1100° C. to avoid loss of adhesion between the PSG and the siliconnitride (Si₃N₄). This entire PSG layer is then removed by etchingleaving the second structural Polysilicon layer exposed for the finalfourth mask. The structures are achieved in an anisotropically patternedin a CCl₄ plasma by reactive ion etching process in order to achievenearly vertical sidewalls. Lastly, the sacrificial PSG is etched awayrevealing the final “floating” or “tunneled” geometry necessary for themicro-machined element. Refer to FIG. 4 of the Resonant MicrostructurePaper for drawings of this surface micro-machining technology. Once thestructure is sealed appropriate packaging can be employed to allowconnection to a vacuum source.

[0066] There are clearly a variety of modifications that could be madeto the above-described invention without departing from its essentialprinciples. It is intended that all such modifications be encompassedwithin the scope of the following claims.

What is claimed is:
 1. A method of reducing the temperature sensitivityof an instrument, comprising the steps of: applying heat to theinstrument; measuring a parameter that is indicative of the temperatureof the instrument; and controlling the amount of heat applied to theinstrument to maintain the measured parameter substantially constant. 2.The method of claim 1 wherein the heat is applied to atemperature-sensitive element of the instrument by means of anelectrical current passed through a resistive element.
 3. The method ofclaim 2 wherein the parameter is the resistance of thetemperature-sensitive element.
 4. The method of claim 1 wherein theparameter is the output of a temperature measuring device.
 5. Amicrobalance comprising: a base; an oscillating element coupled at oneend thereof to the base; and a heater to apply heat to the oscillatingelement.
 6. The microbalance of claim 5 wherein the heater is aresistive element.
 7. The microbalance of claim 6 wherein the resistiveelement is. provided on a surface of the oscillating element; embeddedin the oscillating element, or partially embedded in the elasticelement.
 8. The microbalance of claim 5 wherein the heater is a radiantheater.
 9. The microbalance of claim 5 further comprising controlelectronics to measure a parameter that is representative of thetemperature of the oscillating element, and to control the heater tomaintain the parameter substantially constant.
 10. The microbalance ofclaim 6 further comprising control electronics to measure a parameterthat is representative of the temperature of the oscillating element,and to control the heater to maintain the parameter substantiallyconstant.
 11. The microbalance of claim 10 wherein the parameter is theresistance of the resistive element.
 12. A method of operating amicrobalance, the microbalance including a hollow oscillating elementhaving a filter mounted at one end thereof, the method comprising:inducing resonance in the oscillating element; passing a fluid throughthe filter and through the oscillating element; measuring the resonantfrequency of the oscillating element to determine the mass of materialcaptured in or on the filter; measuring a parameter of the fluid beingdrawn through the oscillating element; and compensating for thevariation in the resonant frequency that has resulted from the variationof the parameter.
 13. The method of claim 12 wherein the parameter isselected from the group consisting of temperature, pressure and density.14. A method of manufacturing a microbalance, comprising the steps of:providing a form onto which an oscillating element can be electroformed;electroforming the oscillating element onto the form; and removing theoscillating element from the form.
 15. The method of claim 14 furthercomprising the step of: forming an insulating layer on the oscillatingelement.
 16. The method of claim 15 further comprising the step of:providing a resistive element on the insulating layer.
 17. A method ofmanufacturing a microbalance, comprising the steps of: depositing aspacer layer on a piece of silicon; layering a microstructure layer ontop of the spacer etching the microstructure layer to define a resonantmicrostructure; and etching the spacer layer thereby to make theresonant microstructure freestanding.
 18. The method of claim 17 furthercomprising the step of: forming a motion sensing layer that usesproximity detection by capacitance or reluctance.
 19. The method ofclaim 18 further comprising the step of: providing an excitationstructure comprising an electrostatic comb drive to drive the resonantmicrostructure.
 20. The method of claim 18 wherein the resonantmicrostructure includes an impaction plate or a filter holder.